1. Field of the Invention
The invention relates in general to a memory and method for charging a word line thereof, and more particularly to a memory and method for charging a word line thereof, which can reduce charging time of the word line.
2. Description of the Related Art
FIG. 1 is a schematic diagram of word-line driving of a conventional flash memory. The conventional flash memory 100 includes at least a word line driver 110 and a word line 120. The word line driver 110 is connected to an operational voltage GWL and controlled by an address-control signal PP. The word line 120 is connected to an output terminal of the word line driver 110. In a read or program operation, the word line driver 110 is controlled by the control signal PP to charge up the word line 120 for supplying the operational voltage GWL to memory cells (not shown in the figure).
The time constant of the charging period is RC wherein R is the total resistance (R1+ . . . +Rn) of the word line 120 and C is the total capacitance (C1+ . . . +C(n+1)) of transistors connected to the word line 120. The charging time of the word line 120 is about 3˜5 times of the time constant RC. Conventionally, the word line 120 is made of poly material, and the poly resistance R of the word line 120 is normally more than 30KΩ. Therefore, the high poly resistance R of the word line 120 will greatly lower down the charge speed of the word line 120 and thus reduce the speed of the read/program operation.
To solve the above issue, traditionally, a metal strap (not shown in the figure) may be formed between a start terminal and an end terminal of each word line 120 so as to reduce the resistance of the word line 120. However, along with the tendency that the memory is designed to be thin and small, the pitch of the word line 120 is narrowed such that there is no enough space to put the strap metal for each word line 120.